The controller structure for many observer-based controllers is the same for numerous actuator sizes. Control parameters are typically based on physical parameters such as torque per ampere, coil resistance, coil inductance, friction, and inertia. For a particular actuator model, unit-to-unit variation of physical parameters is such that all but friction and inertia control parameters are fixed values for each actuator model. Inertia and friction vary depending on what load is attached to the actuator. This is not a big problem for gear-drive actuators because the load inertia as seen by the actuator is reduced by the square of the gear ratio.
However, direct drive actuators generally have lower cost and better reliability than gear-drive actuators. The inertia and friction variation is a problem with direct drive actuators because there is no gear ratio to provide a mechanical advantage. Friction and inertia parameters can be set in a factory type setting when the actuators have an actuator (or other known inertia load) attached at the factory. However, the cost of setting and verifying parameters is only practical for high volume applications due to the costs involved. For other actuators where the load parameters are not known, such as an attachment to a customer's actuator, diesel rack, etc., the inertia parameter needs to be set when installed or a default parameter used.
The problem with using a default parameter is that the allowed range of load inertia is limited. As a result, the potential range of applications for the actuator is reduced when a default parameter is used.
Another possible solution is to make the customer adjust the inertia parameter(s) in the controller. Customer tuning of the inertia parameter has proven to be a problem for numerous reasons. For example, a customer typically does not know the inertia of the load. Tuning the controller by behavior of the actuator is subjective and it does not correspond to the familiar PID (Proportional, Integral, Derivative) gains of which many customers are familiar. This often causes customer confusion and poor actuator tuning.
A further option is to have the controller identify the inertia and tune itself. However, identifying inertia in an observer-based model is difficult because it is located (e.g., hidden) in the middle of the actuator transfer function, is immediately downstream of strong non-linear disturbances (load and friction), and upstream of two integrators. Ideally, net torque and acceleration is measured to determine the inertia parameter setting. However, measuring net torque and acceleration requires sensors that would otherwise not be needed, which results in higher costs and increased board space. In numerous applications, only applied voltage and position is measurable. What is needed is a method to tune the inertia parameter using applied voltage and position measurements.
The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.